1. 6.2-Inheritance of Linked Genes
SBI3U

2. Recall: Mendel’s Observations
Mendel Demonstrated:
Law of Segregation
Law of Independent Assortment
The alleles for a gene segregate independently during meiosis.
This applies to alleles located on homologous chromosomes
This theory was proven to be incorrect.

4. Theory of Linked Genes
See pg Does not show the 9:3:3:1 ratio

5. Theory of Linked Genes
Alleles on the same chromosome do not assort independently.
Some genes that are on the same chromosome are inherited together and are called linked genes.
An example of this additional complex pattern is found in sweet pea plants.

6. Linked genes: on the same chromosome and tend to be inherited together.
Linkage Group:
Genes on a chromosome that are always inherited together (unless crossing over occurs)

7. Crossing Over & Linked Genes
All genes on any one chromosome are called a linkage group b/c they tend to be inherited together
Crossing over can prevent linked genes from staying together.
Ex. 2 genes are on the same chromosome
The alleles for the linked genes switch
Now they are unlinked and will migrate to different gametes

8. UNIT 2
Chapter 6: Complex Patterns of Inheritance
Section 6.2
Linked Genes
Recombinant alleles: linked genes that have been separated during crossing over
In most of the gametes formed, there is no crossing over—they maintain the linkage of the alleles. In a small minority of gametes, crossing over occurs and alleles of previously linked genes become unlinked.

9. Chromosome Mapping Used to determine location of genes on a chromosome
Useful for organisms that reproduce rapidly (e.g. fruit flies)
Frequency of crossing over is connected to how close the pairs of linked genes are
Closer together = low frequency
Further apart = high frequency

10. Sex-Linked Inheritance
Thomas Hunt Morgan studied the fruit fly (Drosphila melanogaster)
He crossed a pure red eyed female with a white eyed male and this is what he found…

11. Genotype
Phenotype
XR XR
Female with red eyes (homozygous dominant)
XR Xr
Female with red eyes (heterozygous)
Xr Xr
Female with white eyes (homozygous recessive)
XR Y
Male with red eyes
Xr Y
Male with white eyes

12. Sex-Linked Inheritance cont’dXR XR
In the F1 generation:
100% of offspring had red eyes. Xr
XRXr
XRXr Y
XR Y
XR Y

13. Sex-Linked Inheritance cont’d
In the F2 generation:
100% of the females have red eyes.
50% of the males have white eyes
50% of the males have red eyes. XR Xr XR
XRXR
XRXr Y
XR Y
XrY

14. Experimental Conclusions
Conclusion: The gene for eye colour is connected to gender and located on the X chromosome.
Traits that are controlled by genes on either the X or Y chromosome are called sex-linked traits.

15. Sex Chromosomes The X chromosome contains 2000 genes
The Y chromosome only contains 100 genes
Sex chromosomes contain some genes that are unrelated to sex determination.
Each sex-linked gene can contain dominant or recessive alleles,

16. Sex-Linked Traits in Humans

17. X-Linked Dominant Vs. X-Linked Recessive
Affected males pass on only to daughters (100% chance)
Females pass to both sons and daughters (100% chance)
Very rare
Ex. Fragile X Syndrome :
Affect more males than females because males only need one allele to be affected, while females need 2 recessive alleles to be affected
Ex. hemophilia, red- green colour vision deficiency
X-Linked Dominant:
X-Linked Recessive

18. X – linked recessive

19. X – linked dominant

20. X-Link Recessive: Colour Vision Deficiency (CVD)
An X-linked recessive trait like CVD will affect more males than females in a family. These males will have difficuty distinguishing between shades of red and green

21. Test Your Understanding…
Determine the probability that a woman who is a carrier for hemophilia and a man without hemophilia will have a child with hemophilia.
What proportion of the female children with have hemophilia?
What proportion of male children will have hemophilia?
Is it possible for these parents to have a female with hemophilia?

22. Test Your Understanding…
A human female with red-green color deficiency (CVD) marries a normal male.
What proportion of their children will have CVD?
Is it possible for the parents to have a female with CVD? Why or why not?

23. Homework:
Read and make notes 6.2
Complete pg. 258 Q# 11-13, 16, 17